1. Field of the Invention
This invention pertains, in general, to improved polyurethane elastomer compositions which are used in contact with phosphate-ester-based hydraulic fluids.
2. Description of the Prior Art
The term "polyurethane elastomers" is generally applied to elastomeric or rubberlike polymers which contain significant numbers of urethane groups, which have the characteristic structure ##STR1## whether the urethane group repeats regularly throughout the macromolecule or not. Such elastomers are ordinarily prepared by the reaction of a polyisocyanate compound with compounds having two or more "active hydrogens." Such active-hydrogen compounds include polyhydroxy compounds, generally termed polyols, and compounds containing amino groups or carboxyl groups. The active hydrogen compound can contain functional groups in addition to groups which supply replaceable hydrogens. Thus hydroxyl-terminated polyethers and polyesters have been used to react with polyisocyanates to prepare polyurethanes, as have polycaprolactone polyols, which contain both ester and ether groups in addition to hydroxyl groups. A detailed discussion of prior-art polyurethane compositions may be found in Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd ed., (Interscience, 1970), Vol. 21, pp. 56-106.
Generally in commercial practices a "prepolymer" technique is utilized to prepare polyurethane elastomers, in which a diisocyanate is reacted with a polyol, usually a hydroxyl-terminated polyester or polyether, to form an isocyanate-terminated prepolymer. The polyols used in forming the polyurethane elastomers generally have molecular weights in the range of about 1,000 to about 3,000. The diisocyanates employed are ordinarily aromatic compounds, because their bulky molecular structure contributes rigidity and tensile strength to the polymer. Two aromatic diisocyanates commonly used in the preparation of polyurethane elastomers are 4,4'-diphenylmethane diisocyanate, designated MDI, and 3,3'-dimethyl-4,4'-diphenyl diisocyanate, designated TODI. A partial listing of other aromatic diisocyanates particularly useful in the preparation of polyurethane elastomers is set forth in Table 16, page 77 of Kirk-Othmer, cited above. During the prepolymer formation step the molecular weight of the material is increased to the range of 25,000 to 50,000.
This prepolymer is then further reacted with a second active-hydrogen compound, ordinarily of lower molecular weight than the polyol used to prepare the prepolymer. This second step is referred to as a "chain extension" reaction, and the second active-hydrogen reactant is termed a "chain extender." Glycol (1,2 ethanediol); 1,4 butanediol; diamines; and trihydroxy compounds have been used as chain extenders. The chain extension reaction causes the segments of prepolymer to join together to produce a very high molecular weight linear material. Chain extension normally does not provide any cross-linking. However, if a trihydroxy compound such as trimethyolpropane is used for chain extension, branching of the polymer will occur.
The above steps are usually carried out at elevated temperatures in the vicinity of about 100.degree. C. and may or may not be catalyzed.
Up to this point the polyurethane polymer exists mainly as a very high molecular weight polymeric material. In order to provide the final physical properties associated with a thermoset elastomeric material, the polymer must be crosslinked. This is accomplished by joining the long polymer chains together through the reaction of free (unreacted) diisocyanate groups on one macromolecule with urethan and/or substituted urea groups on another macromolecule to form allophanate and/or biuret cross-links respectively.
In addition to the prepolymer method discussed above for preparing polyurethane elastomers, a "one-shot" technique is also sometimes used. The one-shot method involves mixing active-hydrogen compounds of different molecular weights and reacting the resultant mixture with a polyisocyanate.
Within the polyurethane elastomer family, it is possible to obtain a wide range of values for the physical and chemical properties of the elastomer by appropriate selection of the specific raw materials, their formulation, or their relative amount within the formulation, as is well known in the art.
Conventional polyurethane elastomers have advantages not possessed by other elastomeric materials; namely: (1) excellent abrasion resistance; (2) higher tear strength; (3) high tensile modulus; (4) high tensile strength at break; (5) outstanding toughness; (6) excellent resistance to oxygen and ozone; and (7) excellent resistance to mineral oil.
Because of their overall good abrasion resistance, high coefficient of friction, low noise level, and particularly their excellent resistance to mineral oils, polyurethane elastomers have found wide use as gaskets, wiper rings, valve seats, and other such seals in hydraulic systems which employ mineral-oil hydralic fluids.
Seals for hydraulic systems made of conventional polyurethane elastomers, however, suffer a serious limitation which has become increasingly important in recent years. Because of the fire hazard which attends the use of mineral-oil hydraulic fluids, users and manufacturers of hydraulic systems often employ phosphate-ester hydraulic fluids in place of mineral oils. Phosphate esters, however, attack conventional polyurethane elastomeric materials, causing them to swell and lose strength. A conventional polyurethane-elastomer seal in contact with a phosphate-ester hydraulic fluid will weaken and have a substantially shortened useful life compared to the same seal in contact with a mineral-oil hydraulic fluid. In high-pressure hydraulic systems employing phosphate-ester hydraulic fluids, there is a significant risk that a seal made of a conventional polyurethane elastomer will fail catastrophically.
Other elastomeric materials, such as certain highly fluorinated polymers, are available which can withstand attack by fluids containing phosphate esters, but generally these materials are significantly more expensive than conventional polyurethane elastomers and, moreover, have a lower abrasion resistance.
I have invented an elastomeric polyurethane composition which is highly resistant to fluids containing phosphate esters, yet retains without significant impairment the advantages of conventional polyurethane elastomers. This composition is particularly adapted for use in fabricating elastomeric seals which avoid the problems associated with conventional polyurethane elastomeric seals in contact with phosphate-ester hydraulic fluids.